1. Field of the Invention
The invention is directed to a method for removal of ATM cells from an ATM communications device, having a plurality of ATM cells, a plurality of which are in each case assigned to a common frame, and which are stored in connection-specific queues B;
2. Description of the Related Art
In conventional packet communications systems, a packet has a comparatively large and variable length. One system for transmitting information in packets with fixed, predetermined lengths is referred to as the ATM (Asynchronous Transfer Mode) system. Such a system allows voice, video and data signals to be processed and transmitted in the same way. The individual packets are normally called cells. The cells each contain a cell header, whose information allows switching and/or assignment of the respective cell. In ATM communications devices, in particular communications network devices, high-speed and broadband transmission is possible at a transmission rate of more than 150 Mb/s.
One problem with ATM communications devices is the level of the transmission rate on a transmission path when a jam of ATM cells has formed there. This problem is described in detail in the German Patent Application 19810058.2. This refers to ATM systems in which a plurality of ATM cells are in each case assigned to a common frame. These frames are data packets of variable length, in a relatively narrow sense. If, for example, a cell in such a frame is lost or has been damaged, it is undesirable for the remaining cells in the same frame to be transmitted further over a transmission path of an ATM device, since the complete information in the frame would no longer be received at the end of the transmission path. The ATM system would thus be unnecessarily loaded dynamically. Particularly when a jam occurs on the transmission path, it is necessary to remove the remaining cells in the frame as quickly and effectively as possible.
It has thus been proposed for ATM cells in a specific frame to be removed in each case when an individual ATM cell arrives at the end of a queue. Such queues are used, in particular, to control a sequence of ATM cells at the end and/or at the start of a transmission path. According to a method which is described in German Patent Application 19810058.2 and which is called Partial Packet Discard (PPD), the first, and if present, other cells in the frame which are already located in the queue are not removed, but only all the newly arriving cells in the frame, with the exception of the last cell of the frame. The PPD method has the disadvantage that at least the first and the last cell in the frame still have to remain in the queue.
German Patent Application 198 400 58.2 discloses a further method, according to which all the cells in a frame, from the first cell to the last cell, are removed from the ATM communications device on arrival in a queue. This method, which is called Early Packet Discard (EPD in the following text), has the advantage that no residual cells remain from a damaged frame, or from a frame which is to be removed for other reasons, and the maximum possible space is thus available for other ATM cells. However, the EPD method cannot be applied to frames whose first cell has already been added to the queue.
The transmission of information using the Internet is an example of communication networks via which information is transmitted in packets with a comparatively large and variable length. The Internet protocol TCP/IP is used in this case, which supports the transmission of frames with a variable length. In practice, these networks have an interface to ATM networks. For this reason, the information contained in data packets has to be converted to ATM cells, and vice versa.
To this end, a frame initial code, for example, is stored which denotes that ATM cell immediately in front of the first ATM cell of the frame in the queue. This information normally exists in the cell header of the last cell of the frame namely, as a rule, in the so-called AAU bit in the cell type field (payload type field) of the cell header. Furthermore, the ATM cells are numbered so that, in the end, the majority of the ATM cells can be assigned to a data packet.
German Patent Application 198 100 58.2 describes a further method for how ATM cells can be removed when overload situations occur in a frame. This method, which is also called the LPD method, is particularly useful when a decision has been made to discard the second part of the frame while the first part is still located in the queue in the ATM system. In this case, the first part of the frame is removed from the queue, and the remaining cells are dealt with in the same way as in the EPD method. Problems always occur with the LPD method whenever cells are stored which cannot be assigned to the frame, in a relatively narrow sense.
These cells include, for example, control cells, OAM cells, or monitoring cells of a general type, which may be introduced at the subscriber end. If these cells are stored in a queue, the relevant frame can no longer be discarded, or can be discarded only with difficulty, in an overload situation.
The downward Remanow A. et al: “Dynamics of TCP Traffic over ATM networks describes a method for removal of ATM cells from an ATM communications device. The already mentioned methods such as Partial Packet Discard or Early Packet Discard are described in this document. However, the document does not address how ATM cells are discarded efficiently in the event of an overload.